BuiltResponse inherits from Response. It only accepts status code as an int to indicate the error). This is quite different from Jersey's implementation where it accepts StatusType/Status so that the server can specify non-generic error status code. This means that the client can only interpret a known error code correctly. When it encounters unknown error code it will simply return "Unknown Code" instead of the error reason phase returned by the server.
It would be nice if RESTeasy could do the same by adding method setStatusType() instead. All the client and server interaction should be updated to provide status reason phase along with the status code accordingly.